1. Field of the Invention
This invention relates to a process of forming a trench or a deep hole, by dry etching, in a semiconductor substrate which is particularly useful for manufacturing an article made from Si, such as semiconductor DRAM device.
2. Description of the Related Art
Trenches formed by selectively digging down through a surface of a semiconductor are necessary to realize high integration and high voltage withstanding properties in a semiconductor device. The configuration of each of the trenches must have a side wall which is a little inclined to form a taper, has a smooth surface, and has a round bottom, as shown in FIG. 1A, from a view point of the manufacturing process and the characteristics of semiconductor devices. Thus, establishment of a technique for forming a trench having a good configuration is required. In FIG. 1A, 101 is silicon substrate and 102 is a side wall angle. Undesirable configurations of trenches are shown in FIG. 1B through FIG. 1H.
Heretofore, dry etching of Si for trenches (the same with deep holes, hereinafter only trenches being referred to) and others are conducted generally by using a fluorine-containing gas (hereinafter referred to as "F system gas"), a chlorine-containing gas (hereinafter referred to as "Cl system gas"), a bromine-containing gas (hereinafter referred to as "Br system gas"), etc., as disclosed in U.S. Pat. Nos. 4,226,665 and 4,208,241. Generally, forming a trench is conducted, as shown in FIGS. 2A through FIG. 2C, by forming an insulating mask of SiO.sub.2 202 over the Si substrate 201 (FIG. 1A), followed by forming a pattern in the mask, and then carrying out etching. Here, a description of further processes for forming integrated circuits is eliminated.
At present, forming of trenches can be performed by an RIE (Reactive Ion Etching) device in an atmosphere of the Br system gas, F system gas, Cl system gas, etc. However, a reliable and stable method as is applicable to commercial production is not yet known. Further, although oxygen gas is mixed for etching polycrystalline Si, it has not been mixed for etching single crystalline Si, because oxygen gas tends to create etching residue and the etching tends to be isotropic.
During forming of a trench, not only the Si substrate 201 is etched, but also the SiO.sub.2 mask 202 is partially etched and becomes thin, as shown in FIG. 2C. The difficulty of mask diminishing is shown by "selectivity ratio against SiO.sub.2 ". The selectivity ratio against SiO.sub.2 is defined as a ratio of etching speed of semiconductor to etching speed of SiO.sub.2. The larger the value thereof, the deeper the trench can be made. This is because at the place where the selectivity ratio against SiO.sub.2 is small, the mask disappears before the trench is etched enough, and the Si of substrate which was under the SiO.sub.2 is eroded. Therefore, an etching gas having a large selectivity ratio against SiO.sub.2 is required. However, F system gas, which produces a highly volatile product, has generally low selectivity ratio against SiO.sub.2, and has a problem that it tends to cause isotropic etching wherein an etching reaction proceed in all direction. Further, generally, F system gas, Cl system gas, and Br system gas, in this order, increase the amount of etching residue. The latter also gives a more ordered side wall configuration.
Prior art, as disclosed in U.S. Pat. No. 4,450,042, provides a general process for etching Si perpendicularly. This process, however, gives trenches of at most 5 .mu.m in depth, and if one tries to etch more deeply, one cannot necessarily form satisfactory trenches having stable configurations and high accuracy, because the configurations of trenches lack uniformity, or if uniform configurations are pursued, much residue is formed. In some cases a mixture of gases are used in order to take advantage of various etching gases. However, an etching process has not yet been proposed which solves all of the problems at the same time, to provide a commercial production.
U.S. Pat. No. 4,784,720 discloses a process of controlling the trench configurations by selectively adhering reaction products onto the side walls of the trenches. However, this process pursues only configuration, and does not pay attention to other properties, e.g. selectivity ratio and etching residue, so that it is difficult to apply the process to mass production.